Metallocenes are of increasing importance as a commercial olefin polymerization catalyst. Typically, a metallocene catalytic system (MCS) is used in the polymerization of olefins. The MCS may comprise a metallocene and an activator on a support, for example, an inorganic support. Such activators are well known and typically include an aluminum alkyl or aluminoxanes, such as methylaluminoxane (MAO). To form a conventional MCS, the metallocene and the optional alumoxane activator may be reacted in the presence of the support to provide a supported metallocene-alumoxane reaction product. For example, a silica gel support may be coated with an alumoxane, such as methylalumoxane (MAO). A metallocene may be complexed with the alumoxane bound to the support to form a MCS that can then be used in an olefin polymerization process. A trialkylaluminum or organoaluminum activator or scavenger maybe employed during the polymerization process to increase catalytic activity.
However, for such MCSs to provide an economically viable alternative to conventional catalysts, a number of limitations must be overcome. For example, the MCS must be capable of producing polymers of the desired stereospecificity and morphology. For example, stereoregular polymers produced from such MCSs should have a certain desired tacticity. Isotactic polypropylene (iPP) or syndiotactic polypropylene (sPP), for example, can be described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units oriented on the same side, or alternating sides for sPP, of a hypothetical plane through the main chain of the polymer.
Desirable morphologic properties may include polymers comprising uniform compact generally spherical particles, having a particular particle size distribution, or a certain bulk density, and low content of fine particles. The generation of undesirable fine particles (i.e., particle diameter less than about 106 microns) can cause plant process difficulties, such as plugging filters, and affect the accuracy of level gauge readings. Alternatively, large particles (i.e., having a low bulk density) are also undesirable because they require more power to circulate though loop reactors, leading to high power consumption and lower production rates.
Additionally, MCSs should ideally have high catalytic activity. One limiting factor in the production of MCSs with high activity is thought to be the low amount of activator or metallocene loaded onto to the support. Another factor limiting catalytic activity is thought to be the low amount of activated metallocene loaded onto the support. Moreover, as the costs for metallocene or activator can be substantial, their efficient use is important to controlling the total cost of producing a MCS.
Accordingly, what is needed in the art is a MCS that provides improved activity, and yet still having acceptable morphological properties, while overcoming the above-mentioned problems.